The present invention relates to a new and improved method of determining the autocollimation angle of a grating coupler.
In its more particular aspects, the present invention specifically relates to a new and improved method of determining, with respect to a light beam, the autocollimation angle of a grating coupler comprising a waveguiding structure which defines predeterminate waveguiding modes in forward and rearward propagation directions, and a diffraction grating having a predeterminate grating period and defining predeterminate diffraction orders.
Grating couplers, prism couplers and endfire couplers are known technical means for exciting a waveguiding mode in a waveguide. When using a grating coupler or a prism coupler, the effective refractive index of a waveguiding mode can be determined by determining the associated incoupling angle. Such incoupling angle is defined as the difference between the angle of incidence for resonance excitation of the waveguiding mode and the autocollimation angle at which the light beam, for example, a laser beam, is reflected back on itself. In this manner, the angle of incidence can be determined very precisely because the excitation of the waveguide mode is based on a resonance phenomenon.
According to one hitherto known method of determining the autocollimation angle, the waveguide to be investigated is mounted on a turntable and a laser beam is directed normal or perpendicular to the surface of the waveguide to be investigated. The turntable is adjusted such that the laser beam is reflected back on itself as precisely as possible. This method, however, is afflicted with relatively wide tolerances. Also, the precise determination of the autocollimation angle is fraught with difficulties and requires an expensive optical autocollimation system.